The use of metallic flake pigments such as aluminum flake pigments in coating compositions used in particular for exterior finishes for automobiles and trucks to provide the finish with metallic glamour is well known. There are relatively few problems with the addition of these metallic flakes by conventional methods to solvent based coating compositions but in waterborne compositions, the metallic flake, in particular, aluminum flake, reacts with water and any acid constituents present in the coating composition and the flake deteriorates and can cause the evolution of gas and finishes formed with such coatings have a reduced brightness.
To avoid such problems, phosphated linear random polymers have been used. Such polymers have been added to metallic flake containing coating compositions as shown in Frangou U.S. Pat. No. 4,675,358 issued Jun. 23, 1987 and in Chang U.S. Pat. No. 5,104,922 issued Apr. 14, 1992. The phosphated portion of the polymer provides passivation of the flake. Residual phosphoric acid groups attached to the polymer are neutralized with an amine or an inorganic base to disperse the polymer into water. These polymers must be sufficiently hydrophobic in order to associate with the metallic flake which typically is an aluminum flake whose surface area is hydrophobic. To obtain a balance of properties with these polymers has been very difficult. If more passivation is needed for the flake, the phosphated portion of the polymer is increased but at the expense of the hydrophobic portion of the polymer which reduces the dispersibility of the polymer. On the other hand, if more dispersibility is needed, the phosphated portion of the polymer is reduced but protection provided to the flake is reduced proportionately. Optimum passivation and dispersibility cannot be obtained since these properties of passivation and dispersibility of the polymer are being balanced against one another. These phosphated polymers offered some improved protection to metallic flake pigments against the evolution of gases and did improve the stability of coating compositions formulated with metallic pigments but additional improvements for long term stability and prevention of gassing are still required.
A polymeric dispersant is needed that will form an aqueous pigment dispersion that is stable, will protect and not allow for the deterioration of the metallic flake pigment, will not allow for the formation of gases and is compatible with a variety of polymeric film forming binders conventionally used in water based coating compositions and that will cure with the film forming binder to form a finish of automotive quality that does not deteriorate on weathering because of adverse properties of the polymeric dispersant.